PROCEEDINGS, Twenty-Seventh Workshop on Geothermal Reservoir EngineeringStanford University, Stanford, California, January 28-30, 2002SGP-TR-171

FLUID-INCLUSION GAS CHEMISTRYOF THE DIXIE VALLEY (NV) GEOTHERMAL SYSTEM

Susan Juch Lutz1, Joseph N. Moore 1, Nigel J.F. Blamey 2, and David I. Norman2

1.Energy & Geoscience Institute, University of Utah, Salt Lake City, UT 841082. Dept. of Earth and Environmental Sciences, New Mexico Tech, Socorro, NM 87801

e-mail: sjlutz@egi.utah.edu

ABSTRACT

Hydrothermally altered samples from outcrops alongthe eastern Stillwater Range (NV), and scale and veinsamples from Dixie Valley geothermal wells werecollected for fluid-inclusion analyses. Fluid-inclusion microthermometry and gas analysis byquadropole mass spectrometry were applied toestablish the chemistry of the fluids trapped duringalteration. Relationships between CO2/CH4, N2/Ar,and H2S were used to evaluate the origins of theinclusion fluids. Most geothermal vein samples fromthe wells are interpreted as mixtures of shallowmeteoric and evolved meteoric (“crustal”) fluids.Fluid-inclusion gases from epidote-bearing faultgouge appear to have a strong crustal signature (withlow CO2/CH4 ratios). Hematite-bearing veinassemblages exhibit gas compositions that areoxidized (with high CO2/CH4 ratios), and meteoric(N2/Ar = 50-100) in origin. Analyses with highN2/Ar ratios (up to 300) indicate a magmatic originfor some fluid-inclusion gases. Actinolite-bearingveins associated with Miocene-age basaltic dikescontain mixtures of magmatic and meteoric gases.There also appears to be a small magmaticcomponent to the gases in quartz-calcite veins fromproduction wells. This result was unexpectedbecause the Dixie Valley system is thought to be adeep-circulation, nonmagmatic geothermal system.However, vapor-rich fluid inclusions in productionwell scales and production fluids also contain somemagmatic helium, and have slightly higher N2/Arratios than gases with a purely meteoric origin.

INTRODUCTION AND GEOLOGIC SETTING

The Dixie Valley geothermal system in west-centralNevada is a typical deep-circulation, fault-relatedgeothermal system. Wells a few kms south of thegeothermal field produce the hottest (265- 275ºC)fluids in the entire Basin and Range Province(Benoit, 1994; Blackwell et al., 2000). Permeablezones in production wells in the field are associatedwith the Dixie Valley fault, a large-displacementnormal fault with a long history of activity. In the

southern Stillwater Range, up to 6 km of verticaldisplacement has been recorded along this faultsystem since the mid-Miocene (Parry et al., 1991).The Dixie Valley fault continues to be one of themost active fault systems in the Basin and RangeProvince (Bell and Katzer, 1987; Caskey et al., 1996;2000a; Caskey and Wesnousky, 2000b). The fluidsin the Dixie Valley geothermal system aredominantly meteoric in origin, and the source of theheat for the geothermal system is thought to be thedeep circulation of fluids in an area of above averagegeothermal gradients (Williams et al, 1997;Blackwell et al., 2000). Isotopic studies of DixieValley production fluids have demonstrated that thefluids are essentially unexchanged meteoric water oflate Pleistocene age (Nimz et al., 1999). However,gases collected at the wellheads appear to contain asmall component of magmatic fluid. Elevated heliumcontents and helium isotopic signatures suggest thatas much as 7.5 % of the helium may have a magmaticsource. Therefore, at least some of the heat in theDixie Valley geothermal system may be magmatic inorigin (Kennedy et al., 1996).

In this paper, we examine the sources and chemistryof fluids trapped in alteration minerals and wellborescales associated with the geothermal system andrecorded by the fluid-inclusion gas compositions.The samples collected for analysis represent a varietyof depth intervals, mineralogies, and alteration types.Outcrop samples collected from the Stillwater rangefront include silicified gouge from the Dixie Valleyfault, corrensite-rich clay from range front faultsplays, geothermal veins with variable amounts ofquartz, calcite, dolomite, barite and hematite,fumarolic encrustations (gypsum), and fossil hotspring sinters and quartz breccias from the DixieComstock Mine (Vikre, 1994; Fig. 1). Forcomparison, actinolite-bearing veins associated withMiocene-age basaltic dikes were also sampled. Inall, thirteen outcrop samples were analyzed, resultingin 133 fluid-inclusion gas analyses. Subsurfacesamples consist of vein fragments hand-picked fromthe cuttings of three geothermal wells and carbonate

scales from four production wells. These elevensamples yielded 93 gas analyses. The objectives ofthis study were to determine the importance anddistribution of magmatic, crustal or meteoric derivedvolatiles during the evolution of the geothermalsystem.

METHODS

Major and minor gases, including H2O, CO2, CH4,H2S, H2, N2, Ar, and C2-7 organic species contained ininclusions in alteration minerals and calcite scaledeposits were analyzed with a Balzers QME 125quadrupole mass spectrometer after being releasedfrom the inclusions by crushing (crush-fast-scan(CFS) method). Norman et al. (1996) have presentedthe details of this analytical technique.

The CFS method involves opening inclusions with aswift crush in a vacuum chamber housing the massspectrometer. The volatiles are removed by thevacuum pumping system in 1 or 2 seconds andrecorded by operating the quadrupole in a fast-scanmode with measurements taken every 150 to 225milliseconds. Opening a 10-20 micron inclusion, orgroup of smaller inclusions of equivalent volume,provides the ideal amount of volatiles for CFSanalysis. Five to 20 crushes can be made on a 0.2 gsample. Species routinely recorded are H2, He, CH4,H2O, N2, O2, H2S, Ar, C3H8, H2, CO2, and SO2. Theprecision of the CFS analyses is <5 % for majorgaseous species and about 10% for the minor species.

ALTERATION MINERALOGY

A long history of hydrothermal alteration is recordedin mineral assemblages exposed along the surfacetrace of the Dixie Valley fault. In southern DixieValley, alteration minerals and fluid-inclusioncharacteristics record over 25 my of hydrothermalalteration and the presence of a variety of differentfluids along the fault, with salinities ranging from 0.1to 39.2 wt % NaCl equivalent, homogenizationtemperatures from 120° to 400°C, and variable CO2

contents (Parry et al., 1991). In northern DixieValley, hydrothermal alteration along the faultappears to be much younger. Here, the fluidinclusions record temperatures up to 325°C (butmostly below 250°C) and contain low salinity (<1.9wt % NaCl equivalent), gas-poor liquids (Fig. 2, Lutzet al., 1998). Production fluids that power the 62MW geothermal plant have temperatures near 240°Cand salinities of 0.1-0.2 wt %. Based on isotopic andgeochemical analyses, these geothermal fluids appearto be less than 13,000 years old (Nimz et al., 1999).Six hydrothermal alteration assemblages can berecognized in the production well cuttings samples.These assemblages consist of (from oldest toyoungest): 1) epidote-chlorite-calcite; 2) illite

(sericite); 3) wairakite-quartz-calcite; 4) mixed-layerillite/smectite and calcite-quartz; 5) chalcedony-dolomite-calcite-barite-chlorite/smectite-hematite;and 6) quartz-calcite (Lutz et al., 1997; 1998). Thevariety of mineral assemblages suggests thatchemically-distinct fluids from different sources havebeen active along the fault system at different times.

In general, four geothermal alteration assemblagesare found in outcrops along the eastern Stillwaterrange front adjacent to the geothermal field: 1)calcite-dolomite-hematite-barite veins and travertinedeposits; 2) quartz-rich fault breccias and sinterdeposits; 3) travertine and calcite veins, and 4)gypsum-kaolin-halite fumarole encrustations. Thesinters are fossil hot spring deposits that occur alongthe surface trace of the Dixie Valley fault in an areawhere fumaroles are now active (see Section 10/15area in Fig. 1; Lutz et al., this volume). Themineralogy of the Stage 1 travertine and Stage 2sinter deposits is similar to vein assemblages 5 and 6,respectively, in the geothermal reservoir (seeprevious discussion) and suggests that these aresurface-subsurface expressions of the samehydrothermal event at different depths along theDixie Valley fault. It can be difficult to tie individualcalcite vein fragments in the well cuttings to thesestages of alteration. However, samples from well 52-18 at 8030 and 9300 ft (2450 and 2835 m) containdolomite, hematite and barite and probably representStage 1 veins. Prismatic quartz from well 73B-7 at8783 and 8876 ft (2677 and 2705 m) may representStage 2 veins; these veins occur in a currentlyproductive splay of the fault system in thisproduction well.

The alteration assemblages represent two majorstages of hydrothermal activity related to the presentgeothermal system. The oldest is the calcite-dolomite-hematite-barite assemblage that appears topredate strong quartz alteration at the DixieComstock Mine (Vikre, 1994), the Mirrors locality,and in geothermal well cuttings (Lutz et al., 1997;1998). Radiocarbon dating of organic material in thetravertine and sinter deposits suggests that the calcite-dolomite-hematite-barite assemblage may be about5040 years old, whereas young opaline sinters (andassociated subsurface quartz veins) may range in agefrom modern to 3450 years old (Lutz et al., thisvolume).

FLUID-INCLUSION GAS CHEMISTRY

Fig. 3 shows the results of fluid-inclusion gasanalyses for selected outcrop and well cuttingssamples. The data are plotted on a diagramdeveloped by Norman et al. (1998) that usesCO2/CH4 vs. N2/Ar to differentiate fluid inclusiongases into fields that include magmatic, shallow

meteoric, evolved meteoric (crustal), and crustal-organic zones.

CO2/CH4 - N2/Ar Relationships

Norman et al. (1996) demonstrated that the ratios ofCH4, N2, and Ar are useful indicators for tracing thesources of gases trapped in fluid inclusions. Theyargued that hydrothermal fluids derived frommeteoric waters will have N2/Ar ratios between thoseof air (84) and air-saturated water (36) althoughboiling will expand this range slightly because ofdifferences in the solubilities of the two gases.Meteoric fluids that have boiled can have N2/Arratios that range from about 100 to 15, depending onwhether the inclusions trapped the gas-depletedliquid or the steam (Norman et al., 1997). Fluidsfrom active Basin and Range geothermal systemshave N2/Ar ratios up to about 100 or 150 (Welhan etal., 1988). Crustal fluids (those which are notinvolved in the meteorological cycle) are typicallyenriched in CH4 and other hydrocarbons.Hydrothermal fluids may accumulate CH4 throughthermal degradation of organic material, and/oroxidation-reduction reactions involving iron-bearingminerals and CO2 (Giggenbach, 1992). Magmaticgases are distinguished by low CH4 contents, high3He/4He isotopic ratios, and N2/Ar ratios exceeding100 (Giggenbach, 1986; Norman and Musgrave,1995; Norman et al., 1996).

The fluid-inclusion gas chemistry of the Dixie Valleysamples in Figure 3 indicates the mixing betweenthree end-member fluids; shallow meteoric, evolvedmeteoric, and magmatic. Most vein samples from thegeothermal wells are interpreted as mixtures ofshallow meteoric and evolved meteoric (“crustal”)fluids that have N2/Ar ratios between 40 and 100.Hematite-bearing vein assemblages exhibit gascompositions that are oxidized (with CO2 contents of4.4 mol % and high CO2/CH4 ratios), and meteoric(N2/Ar = 50-100) in origin (Fig. 4). Fluid-inclusiongases from epidote-bearing fault gouge appear tohave the strongest crustal signature with highmethane contents (up to 2.2 mol % CH4) and lowCO2/CH4 ratios (Fig. 5). Actinolite-bearing veinsassociated with Miocene-age basaltic dikes containmixtures of magmatic and meteoric gases (Fig. 6).Analyses with high N2/Ar ratios (up to 300) indicatea magmatic origin for these fluid-inclusion gases.

Gases from several of the samples from thegeothermal wells record mixing between crustal andmagmatic fluids. Quartz veins from 9750 ft (2972 m)in well 52-18 and quartz-calcite veins from 8783 ft(2677 m) in well 73B-7 (Fig. 7) show a mixing trendbetween evolved meteoric fluids with high CH4

contents (as much as 1 mol %) and magmatic fluidswith high N2/Ar ratios (up to 276). The vein samplefrom well 73-7 consists of very fine prismatic quartz

that appears to have been deposited in a productivefault which is parallel to the main range front fault.The fluid inclusions in these quartz crystals mayrepresent modern geothermal gases.

In the following sections we relate the differences ingas compositions to the effects of mixing betweenfluids of different origins.

He- N2 -Ar Relationships

The helium compositions of geothermal and volcanicsystems have been studied by Giggenbach (1992,1995). In these studies, the end-membercompositions for various types of volcanic gaseswere related to their geological setting (e.g. andesiticor basaltic volcanoes and their associated geothermalsystems). Basaltic systems are characteristic of rift(New Zealand, Iceland) and hot spot (Hawaii)tectonic settings. The andesitic systems are from avariety of continental and oceanic convergent plateenvironments associated with subduction complexes.Giggenbach attributes a large range in andesite gascompositions and high N2 contents to assimilation ofmarine sedimentary crust during ascent of themagma. In both andesitic and basaltic systems,geothermal fluids (both liquid and gases) mix withair-saturated groundwaters (ASW in Fig. 8).

Welhan et al. (1988) studied the gas compositions ofBasin and Range geothermal systems and basaltic toandesitic volcanoes from western North America.The study revealed that the gases from Basin andRange hot-spring systems follow the basalt orcontinental line that ranges from predominantlymeteoric in origin, to more evolved fluids that havehigh He contents (Fig. 8). An analysis of theBeowawe (NV) geothermal field was included. Thisanalysis plots near the He corner of the He-N2-Ardiagram, but with much lower helium contents thanmost magmatic or volcanic-related geothermalsystems (e.g., Coso; Lutz et al., 1999; Adams et al.,2000).

Because He can be produced both as primaryemissions from the mantle and radiogenically in thecrust, 3He/4He isotopic ratios can be used todistinguish between a mantle or crustal origin. Ratiosnormalized to atmospheric abundances that aregreater than 5 are generally considered to have asignificant amount of mantle He. In general, heliumisotope ratios of geothermal systems within thecentral Basin and Range vary from 0.1 to 2.5 Ra andreflect the non-magmatic nature of these geothermalsystems. For example, the helium isotopic ratio forfluids from the Beowawe geysers is 0.46 Ra,compared to a ratio of 7.08 Ra for the Devil’s Kitchenfumaroles at the Coso geothermal field (Welhan etal., 1988).

A survey of noble gas abundances and isotopiccompositions of Dixie Valley production fluids wasconducted by Kennedy et al. (1996). Their studydetermined that the helium component of productionfluids has an isotopic composition of 0.70 to 0.76 Ra,

which suggests that ~7.5 % of the helium in thereservoir fluid may be mantle derived. Kennedy etal. suggested two mechanisms for the modestenrichment of mantle He in the Dixie Valleygeothermal system: 1) fluid circulation through anaged and non-active magma chamber (perhaps thesource chamber for local Miocene basalts), and 2)fluid transport along the range front fault from deepmantle sources.

Dixie Valley Reservoir Gas Compositions

The gas composition of fluids from Dixie Valleyproduction well 76-7 (data from T. DeRocher, 2002)is plotted on the CO2/CH4 vs. N2/Ar diagram inFigure 3 and on the He- N2-Ar diagram in Figure 8.The analysis falls in the shallow meteoric portion ofFigure 3 and in the basaltic or continental area nearthe Beowawe analysis in Figure 7. In Figure 3, theanalysis of the produced gas plots near fluid-inclusion gases from various carbonate veinassemblages but with a slightly higher CO2 content.

Fluid-inclusion Gas Geothermometry

The proportions of CO2, CH4, and H2S in the fluid-inclusions can be used as a gas geothermometer aslong as several assumptions are held. Theassumptions are: the gases are in equilibrium at thetime of trapping, homogeneous trapping of a singlefluid, no boiling has occurred, that species have notreacted nor were lost after trapping, and that the fluidsalinity is known (Blamey and Norman, this volume).The salinities of Dixie Valley geothermal fluids aregenerally very low (see Fig. 2), and this method ofgas thermometry can be applied. Figure 9 illustratesthe fluid-inclusion gas equilibria for selected DixieValley alteration samples. In this low-sulfur system,most of the geothermal veins plot in the magnetitestability field at temperatures below about 200°C.The gases in actinolite-bearing veins from theMiocene dike are in equilibrium with magnetite andpyrite at about 230°C. For geothermal veins, the gasfluid-inclusion equilibrium temperatures are about50° less than both fluid-inclusion homogenizationtemperatures (see Fig. 2), and present productiontemperatures (~240°C).

Another application of fluid-inclusion gasthermometry is in the interpretation of equilibriumconditions during alteration. The gases in thegeothermal veins appear to have approachedequilibrium from the cool side of the diagram (inother words, these veins were deposited from fluids

undergoing heating; see Norman et al., 1998 for amore complete discussion). This interpretation isconsistent with a deep-circulation model for theheating of Dixie Valley geothermal fluids.

SUMMARY

The compositions of fluid-inclusion gases indicatethat fluids of different origins were involved in theformation of alteration minerals during the evolutionof the Dixie Valley geothermal system.

Epidote-bearing assemblages are characterized byfluid-inclusion gases with high methane contents andlow CO2/CH4 ratios. This assemblage appears torecord reduced conditions along the Dixie Valleyfault at depths greater than 2-3 km. The source of themethane in these crustal fluids may either be from thepyrolysis of sedimentary organic compounds or theresult of Fisher-Tropsch reactions. N2/Ar ratios of 61to 104 suggest that fluids along the fault haveevolved from meteoric fluids, and are not related toeither mantle gases or magmatic fluids associatedwith Miocene basaltic dikes.

Most of the carbonate vein assemblages appear tohave formed from the heating of shallow meteoricfluids. The presence of vein minerals with retrogradesolubilities, such as calcite, dolomite and barite, isconsistent with this interpretation. The highCO2/CH4 ratios are also consistent with thedeposition of hematite from oxygenatedgroundwaters. The presence of minor amounts ofchalcedonic quartz and the occurrence of dolomiteindicates formation of these veins below 180°C(Fournier, 1985; Browne, 1993), which is consistentwith the equilibrium fluid-inclusion gasgeothermometry.

Prismatic quartz veins contain inclusions that aremost similar to fluids in the Dixie Valley geothermalreservoir. The gas chemistry of the quartz veinssuggests mixing between two distinct fluid sources,an evolved meteoric fluid and a magmatic fluid.Most of the quartz-bearing vein assemblages fromoutcrop and well cuttings samples have N2/Ar fluid-inclusion gas ratios that are above 100 and appear tocontain a small component of magmatic gas.

The source of this magmatic component is notcompletely understood; no shallow magmatic bodieshave been identified seismically or by other means inthe Dixie Valley region. The youngest volcanicrocks in the area are late Miocene basalts (K-Ar ageof ~8.5 my; Weibel, 1987) that are unlikely to haveretained any radiogenic helium. Kennedy et al.(1996) have modeled the migration of helium from adeep (~30 km) mantle source up along the DixieValley fault. However, the fluids associated with

epidote-bearing fault gouge at 2.8 km appear to beentirely non-magmatic. Kennedy et al. havesuggested that an isolated, aged magma chamber maypossibly supply magmatic gases from U and Thdecay. The fluid-inclusion gas chemistry seems toimply that Miocene basalt could be the source ofmagmatic gas in the geothermal veins.

ACKNOWLEDGEMENTS.

We would like to thank Caithness Energy and theCaithness-Dixie Valley staff for their support duringfieldwork sessions. Thanks also to Ted DeRocher forpermission to use data from his PhD dissertation. Wealso appreciate Mike Adams` help with interpretingthe gas data and drafting by Doug Jensen. Fundingfor this study was provided by the U.S. Departmentof Energy, for SJL and JNM under Contract DE-AC07-95ID13274, and for DIN and NB underContract DE-FG07-00ID13953.

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